let find_pos map =
  let rec impl row =
    if row >= Array.length map then failwith "find_pos"
    else
      match Array.find_index (fun x -> x = '^') map.(row) with
      | Some i -> (i, row)
      | None -> impl (row + 1)
  in
  impl 0

let read_file fname =
  let lst = Aoc.strings_of_file fname in
  let map1 = Array.of_list lst in
  let map2 =
    Array.map (fun s -> Array.init (String.length s) (String.get s)) map1
  in
  let pos = find_pos map2 in
  (map2, pos, (0, -1))

let is_valid_pos map (x, y) =
  if y < 0 || y >= Array.length map then false
  else if x < 0 || x >= Array.length map.(y) then false
  else true

let rec move (map, (x, y), (dx, dy)) =
  let () = map.(y).(x) <- 'X' in
  let x', y' = (x + dx, y + dy) in
  if is_valid_pos map (x', y') && map.(y').(x') = '#' then
    move (map, (x, y), (-dy, dx))
  else (map, (x', y'), (dx, dy))

let part1 (map, pos, vel) =
  let rec impl (map, pos, vel) =
    if is_valid_pos map pos then impl (move (map, pos, vel)) else (map, pos, vel)
  in
  let _ = impl (map, pos, vel) in
  Array.iter
    (fun y ->
      Array.iter (fun x -> print_char x) y;
      print_newline ())
    map;
  Array.fold_left
    (fun acc y ->
      Array.fold_left (fun acc x -> if x = 'X' then acc + 1 else acc) acc y)
    0 map

let _ = Aoc.main read_file [ (string_of_int, part1) ]